image_serializer.rb -- ArgumentError: bad argument (expected URI object or URI string)

using s3 for assets, add_host_prefix error

Starting pushing objects to Wombat
Pushed 1 Spree::Order to Wombat
rake aborted!
ArgumentError: bad argument (expected URI object or URI string)
/var/app/containerfiles/vendor_bundle/ruby/2.0/bundler/gems/spree_wombat-78d5ed2919da/app/serializers/spree/wombat/image_serializer.rb:31:in `add_host_prefix'
/var/app/containerfiles/vendor_bundle/ruby/2.0/bundler/gems/spree_wombat-78d5ed2919da/app/serializers/spree/wombat/image_serializer.rb:9:in `url'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:124:in `block in attributes'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:123:in `each'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:123:in `each_with_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:123:in `attributes'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:187:in `serializable_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/array_serializer.rb:41:in `block in serializable_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/activerecord-4.1.2/lib/active_record/relation/delegation.rb:46:in `map'
/var/app/current/vendor/bundle/ruby/2.0/gems/activerecord-4.1.2/lib/active_record/relation/delegation.rb:46:in `map'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/array_serializer.rb:40:in `serializable_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:173:in `serialize'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:136:in `block in associations'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:131:in `each'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:131:in `each_with_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:131:in `associations'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializer.rb:188:in `serializable_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/array_serializer.rb:41:in `block in serializable_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/activerecord-4.1.2/lib/active_record/relation/delegation.rb:46:in `map'
/var/app/current/vendor/bundle/ruby/2.0/gems/activerecord-4.1.2/lib/active_record/relation/delegation.rb:46:in `map'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/array_serializer.rb:40:in `serializable_object'
/var/app/current/vendor/bundle/ruby/2.0/gems/active_model_serializers-0.9.0.alpha1/lib/active_model/serializable.rb:9:in `as_json'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:90:in `jsonify'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `block in jsonify'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `each'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `map'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `jsonify'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:88:in `block in jsonify'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:88:in `map'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:88:in `jsonify'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `block in jsonify'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `each'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `map'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:86:in `jsonify'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:34:in `encode'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/json/encoding.rb:21:in `encode'
/var/app/current/vendor/bundle/ruby/2.0/gems/activesupport-4.1.2/lib/active_support/core_ext/object/json.rb:37:in `to_json_with_active_support_encoder'
/var/app/current/vendor/bundle/ruby/2.0/bundler/gems/spree_wombat-78d5ed2919da/lib/spree/wombat/client.rb:30:in `block in push_batches'
/var/app/current/vendor/bundle/ruby/2.0/gems/activerecord-4.1.2/lib/active_record/relation/batches.rb:125:in `find_in_batches'
/var/app/current/vendor/bundle/ruby/2.0/bundler/gems/spree_wombat-78d5ed2919da/lib/spree/wombat/client.rb:24:in `push_batches'
/var/app/containerfiles/vendor_bundle/ruby/2.0/bundler/gems/spree_wombat-78d5ed2919da/tasks/push_it.rake:13:in `block (3 levels) in <top (required)>'
/var/app/containerfiles/vendor_bundle/ruby/2.0/bundler/gems/spree_wombat-78d5ed2919da/tasks/push_it.rake:12:in `each'
/var/app/containerfiles/vendor_bundle/ruby/2.0/bundler/gems/spree_wombat-78d5ed2919da/tasks/push_it.rake:12:in `block (2 levels) in <top (required)>'
Tasks: TOP => wombat:push_it

i think it may have something to do with using a Proc for my asset_host, any ideas?

config.action_controller.asset_host = Proc.new do |source|
  if source =~ /\/pictures/
    nil
  else
    "//s3-#{ENV['FOG_REGION']}.amazonaws.com/#{ENV['FOG_DIRECTORY']}"
  end
end

additionally, the image_serializer being used by Spree::Wombat::ProductSerializer so if i'm not pushing Spree::Product objects then I won't receive the error.